This invention relates to the recovery of underground hydrocarbons in an enhanced oil recovery flood by the use of a premixed transition zone slug composed of carbon dioxide and critical concentrations of crude oil components which give the premixed transition zone slug first contact miscibility with the underground hydrocarbons at formation temperature and a selected pressure substantially lower than the pressure needed for first contact miscibility of carbon dioxide with the underground hydrocarbons.
It has long been known that oil recovery can be improved from most reservoirs by the injection of carbon dioxide into the reservoir. The carbon dioxide generally reduces the viscosity of the hydrocarbons and swells the oil, thereby leading to increased recovery. The general goal of carbon dioxide flooding is to achieve the maximum possible miscibility of the carbon dioxide with the underground hydrocarbons within economic limitations and achieve such miscibility as early as possible in the flood. Unfortunately, injection pressures greater than or equal to 5,000 psi are required to achieve first contact miscibility of carbon dioxide with the hydrocarbons in most underground formations. The cost of achieving such high injection pressures and the inability to use such high injection pressures in many reservoirs prevents the desirable goal of first contact miscibility from being reached at an early stage in the flood. As a result, many carbon dioxide floods have been designed as multiple contact miscibility floods, wherein this miscibility is not achieved until after many contacts of the carbon dioxide with the underground hydrocarbons over a substantial radial distance from the injection well. Such floods are not as efficient as floods in which the carbon dioxide achieves first contact miscibility with the hydrocarbons.
It is also well known in the art to mix various hydrocarbon solvents with carbon dioxide to further lower the viscosity of the underground hydrocarbons and increase the flood recovery efficiency. This approach has been tried with many different mixtures of carbon dioxide and hydrocarbons in attempts to recover more oil. Most solvents used are relatively low molecular weight hydrocarbons ranging from methane to hexane. The flooding efficiencies of such processes are generally greater than flooding with only carbon dioxide, but are achieved at the increased cost of the injected hydrocarbons.
U.S. Pat. No. 2,875,832 discloses a method of injecting carbon dioxide into a formation at a pressure of 300 to 1400 psi and higher, recovering the produced fluids, condensing the produced liquids from the produced fluids and recycling the gaseous effluent into the injection well for increased recovery. Although this method substantially increases the recovery of hydrocarbons, a substantial period of time must pass before first contact miscibility is achieved in the formation and production of fluids is achieved. And years may pass before produced fluids are recovered for injection into the reservoir. The produced fluids must also be mixed together well prior to injection into the ground. Even with substantial mixing, some components of the produced fluids, particularly light ends, will be lost.
U.S. Pat. Nos. 2,875,830, 3,295,601, 3,811,503 and 4,136,738 disclose methods of flooding with mixtures of carbon dioxide and light hydrocarbons in the range of ethane to butane and methane to hexane (for 4,136,738) to achieve conditional miscibility. A cyclical push-pull process is advocated by U.S. Pat. No. 3,295,601 to condition the near wellbore area for the injection well to a miscible state so that a resulting drive fluid can be more effective. U.S. Pat. No. 4,271,905 discloses carbon dioxide combined with naphtha as a flooding medium.
Articles in the Journal of Petroleum Technology of December 1974, p. 1427 and October 1977, p. 1248 discuss the use of mixtures of carbon dioxide and light hydrocarbons for oil displacement. A similar disclosure can also be found in SPE Paper No. 11678 entitled "A Controversial Laboratory Study Of The Mechanism Of Crude Oil Displacement By Carbon Dioxide", presented at the 1983 California Regional Meeting of the Society of Petroleum Engineers of AIME in Ventura, Calif., Mar. 23-25, 1983.
In a study of the miscibility of nitrogen injection systems, entitled "Preliminary Experimental Results of High-Pressure Nitrogen Injection for EOR Systems, Society of Petroleum Engineers Journal, April 1983, p. 339, the authors analyzed the stripping mechanism of nitrogen injected through crude oil. They noted that the primary displacement mechanism was a stripping process which enriched the nitrogen with crude oil fractions of C.sub.1 through C.sub.5 and a small quantity of intermediate crude fractions. It was disclosed that the enrichment of nitrogen vapor continued until miscibility was reached.